1. Field of the Invention
The present invention relates to a method for improving the springback qualities of superconductors. According to the method, in the final stage of production a superconducting wire is cold worked by employing an extremely small reduction, in which case the springback qualities are improved, but the residual resistivity ratio, RRR, still remains sufficiently high.
2. Description of Related Art
In the cabling of Cu/NbTi superconducting wires, springback has become an essential quality, because extremely high requirements are set for the homogeneity of magnetic fields. The term `springback` means the springlike return that takes place in the wire, when it is, after bending, released from the influence of external power. For instance, in the testing instructions of SSC (Superconducting Super Collider), wires there is defined a method for determining the springback qualities of superconductors. In the test, the wire is wound ten times around a bar with a diameter of 9.5 mm, while the tension for the whole duration of the winding is 2 kg. The tension is removed, and the return measured by degrees.
In a cable (for instance of the Rutherford type) which is cabled of a wire with weak springback qualities, the individual wires can rise up from the planar surface of the cable, thus causing changes in dimension. The relatively strong return tendency of bent superconducting wire is due to the springlike property of the superconductor in the final annealed state. The copper serving as the matrix material is normally soft annealed in order to achieve a good RRR value, and soft copper cannot resist the return tendency of NbTi filaments.
It has been attempted to change the springlike return effect by changing the cross-section of the wire by placing the filament area as centrally as possible. This is, however, only possible with such conductors that contain an extremely small amount of some superconducting material. Even then, however, the shifting over to an area with the risk of central bursting in the drawing may easily take place. This restricts the achieved lengths.
When employing a normal production route, the superconductor is in the end annealed in order to make the matrix copper soft, so that a high RRR value is secured at the same time. In some cases (for instance when using large Cu/Sc ratios) there are needed conductors with a higher strength value than the soft copper matrix can produce. In this case the wire is 7-9% overdrawn. The strength increases, but the RRR is decreased nearly down to its minimum. The 7-9% overdrawing is carried out exactly for improving strength, and it is a normal method for this purpose.